A dispensing system is the assembly at a gasoline station that actually delivers the fuel, (e.g. gasoline, diesel fuel, kerosene or alcohol) from a storage tank into the tank of the vehicle in which the fuel is to be used. At most gasoline stations and other locations at which the fuel is dispensed, the fuel is stored in an underground storage tank. The dispensing system includes a pump that draws the fuel from the storage tank to an above ground level elevation so that it will flow into the vehicle fuel tank. In a typical dispensing system, the fuel is pumped from the storage tank, passed through a flow meter and then is delivered to the vehicle through a flexible hose. The flow meter performs a volumetric measurement of the quantity of the fuel that is discharged to provide the data needed to ensure that the customer is accurately charged for the amount of fuel delivered. Often this charge data is presented on a display associated with a data processing unit that also forms part of the dispensing system.
In the past it has been a common practice to design a fuel dispensing system so that it includes a submersible pump that is located in the base of the associated storage tank. When the pump is actuated, it forces the fuel from the tank through a supply line that leads to an above-ground dispenser and from there through a hose through which it is delivered to the vehicle. A disadvantage of this dispensing system is that should there be any leaks in the supply line to the dispenser, the fuel, which is under pressure, is prone to flow out of the tank wherein it can potentially serve as an environmental pollutant.
One solution to this problem is to provide a dispensing system with a suction pump. This type of system is designed so that the pump is located in the actual above ground housing in which most of the other components of the system, the flow meter, the processor and the hose connection, are contained. The pump develops a suction which draws the fuel from the storage tank and then forces the fuel from the pump and through the flow meter and the hose. An advantage of providing a dispensing system with a suction pump is that should there be any leaks in the supply line, the suction drawn by the pump, instead of allowing the fuel to flow out, will draw air into the line. Thus, these type of dispensing systems serve to minimize the unwanted fuel leakage and the attendant enviromental damage such leakage can foster.
While a dispensing system with a suction pump is useful for reducing the likelihood of environmental damage, there is a disadvantage associated with its use. Should a leak be present in the associated supply line, the air drawn into the line by the pump will be compressed in the pump and entrained into the fuel stream that flows through the flow meter and out through the hose. The air is then delivered to the vehicle along with the liquid-state fuel. Consumers, who pay for fuel based on volumetric quantity delivered, take exception to finding they have paid for fuel and have, instead, received a sizeable quantity of air. In many jurisdictions regulatory authorities have placed strict limits on the error rate between the quantity of liquid-state fuel that is measured by the flow meter and the actual amount of fuel actually delivered.
Thus, a typical dispensing system with a suction-type pump is further provided with some type of air separation unit. One such air separation has an elongated, horizontally aligned chamber that is located downstream of the pump. The fuel is introduced in this chamber so as to form a horizontally oriented vortex. Air disposed within the fuel stream as it enters the chamber is supposed to migrate to the center of the chamber where it is forced out through an appropriate duct. While this air separator is of some utility, its overall ability to remove air from the fuel stream in certain situations is open to question.
Moreover, it is further desirable to provide this type of system for monitoring the quantity of air in the fuel stream prior to the fuel stream being applied to the flow meter. This monitoring is desirable because, in the event the quantity of the air is above a given level, the air separator, regardless of its efficiency, may not be able to remove sufficient quantities of air from the fuel stream. When the fuel stream is in this state, the dispensing system should shut down as opposed to continue to operate while possibly delivering inaccurately metered volumes of fuel. In the known prior art system, this monitoring is performed by simply monitoring the pressure of the fuel stream at a point downstream from the air separator and upstream of outlet of the dispensing system. If the fuel stream at this point contains a significant quantity of air, the pressure head at this point should fall. This pressure drop is then interpreted as an indication that the fuel stream contains such a large quantity of air that downstream delivery of the fuel should cease. A potential problem with this type of monitoring system is that the pressure head being monitored may change as a function of flow rates downstream of the point at which the monitoring occurs. In some circumstances, these variations could potentially result in a pressure head developing that does not accurately represent the quantity of air in the fuel stream. Thus a potential exists that the pressure head being monitored will indicate that there is only a minimal amount of air in the fuel stream when, in fact, that is not the case. If this occurs, the dispensing system will continue to allow fuel to be dispensed even though the fuel stream may contain an unacceptably high quantity of air.